The present invention relates to a photosensitive member useful for electrophotography, and more particularly to a composite type photosensitive member for electrophotography which has a high sensitivity even to light having a long wavelength.
A large number of organic substances have been proposed in the past as a charge generating material for a composite type photosensitive member comprising a charge generating layer formed on an electroconductive substrate and a charge transport layer formed on or beneath the charge generating layer. Examples of such known substances include monoazo dyes, diazo dyes, and squaric acid derivatives that are soluble in organic primary amines and are disclosed in Japanese Patent Laid-Open No. 55643/1977, quinocyanine pigments that are disclosed in Japanese Patent Laid-Open Nos. 42380/1978 and 41230/1978, copper phthalocyanine pigments that are disclosed in Japanese Patent Laid-Open No. 11763/1976, and so forth. Inorganic substances such as Te-As-Se have also been known.
On the other hand, various charge transport substances have been also proposed. Examples include polyvinylcarbazoles, pyrazoline derivatives, trinitrofluorenone, various nitro- and cyano-substituted compounds, and so forth.
The photosensitive member for electrophotography using these charge generating and transport materials showed good electrophotographic properties but still have some defects such as no or little sensitivity to the near-infrared light (having a wavelength of 750 nm or longer). Though the known materials are highly sensitive to the light in visible range of from 400 to 700 nm, they are not effective as a photosensitive member for the near-infrared light emitted from a semiconductor laser.
In the past few years, there have been proposed high speed laser beam printers in which semiconductor lasers are used as a light source. The semiconductor lasers can generally emit the near-infrared light. Hence, the development of photosensitive members which have a high sensitivity to a long wavelength light, especially to the near-infrared light, is desired.
By employing the semiconductor laser as a light source, the size of the printer itself can be reduced and a power consumption can also be reduced.
The charge generating substance must have such a property that it forms an electron-hole pair by the light passing through the charge transport layer and implants the resulting hole (or electron) into the charge transport layer by an electric field. On the other hand, the charge transport substance must have such properties that a light carrier generated in the charge generating layer by illumination of light can be injected effectively to the charge transport substance and it has a suitable light absorption range that does not impede the wavelength of light absorbed by the charge generating layer.
A distinct correlation exists between the effective injection of the light carrier and the ionization potential of the charge transport layer. The result of researches has already been reported that in the case in which an electron is the carrier, for example, the electron affinity of the charge transport layer must be higher than that of the charge generating layer and the ionization potential must be lower when a positive hole is the carrier.
On the other hand, a sensitive wavelength range as the photosensitive member depends upon an absorption wavelength range of the charge generating layer unless the charge transport layer to be used interferes with the light absorbed by the charge generating layer. Although various kinds of organic photoconductors are known, their sensitivity is generally limited to the visible range of up to 700 nm.